Conventionally, electronic timepieces are commercially available in which a solar cell is arranged on the dial plate of the timepiece in place of a battery, and the solar cell is combined with an electric double layer capacitor to constitute a power supply, thereby avoiding troublesome battery exchange. However, the output voltage of this power supply using a solar cell varies depending on solar rays or illumination light. More specifically, this power supply is charged upon reception of an optical energy and increases the output voltage. However, when the timepiece load is driven without receiving the optical energy for a long time, e.g., during nighttime, the power is consumed, and the voltage gradually decreases. In the electronic timepiece using this power supply, when the electric double layer capacitor is charged to 2.6 V, and the timepiece load is continuously driven from this state without being charged midway, the output voltage gradually decreases, as shown in FIG. 1.
In a conventional electronic timepiece of this type, the minimum voltage allowing to drive hands as a timepiece load, i.e., the minimum driving voltage is 1.3 V. For this reason, the driving time is t.sub.1, as indicated by V.sub.D in FIG. 1. However, the electric circuit of the electronic timepiece can operate at a lower voltage of about 0.8 V. Therefore, when a large hand driving pulse necessary for driving the hands is independently prepared, the minimum driving voltage can be decreased up to 1.05 V. As a result, the driving time can be increased to t.sub.2, as is apparent from FIG. 1. This means that the electronic timepiece does not stop for a long time even when it is left in an uncharged state, and the charge during the discharge prolongs the driving time, resulting in an increase in product value of electronic timepieces.
From such a viewpoint, an electronic timepiece having a circuit arrangement as shown in FIG. 2 can be considered by applying a pulse width change driving technique disclosed in Japanese Examined Patent Publication No. 61-15386 to an electronic timepiece using a power supply formed of a combination of a solar cell and an electric double layer capacitor.
Referring to FIG. 2, reference numeral 40 denotes a power supply means constituted by a solar cell 1 serving as a power generation means, and an electric double layer capacitor 2 serving as an accumulation means, which serves as a power supply for an electronic timepiece. Reference numeral 4 denotes a quartz oscillation circuit; 5, a time counting circuit; 107, a pulse preparation circuit; and 108, a pulse selection circuit. The pulse preparation circuit 107 and the pulse selection circuit 108 constitute a driving pulse preparation means 109. Reference numeral 11 denotes a driver circuit; 12, a rotation detection circuit; and 13, a stepping motor. The solar cell 1 is arranged on the dial plate of the timepiece to convert an external optical energy into an electric energy. The electric double layer capacitor 2 accumulates the electric energy generated in the solar cell 1 and supplies the power to a timepiece circuit 100 including the quartz oscillation circuit 4, the time counting circuit 5, the pulse preparation circuit 107, the pulse selection circuit 108, the driver circuit 11, and the rotation detection circuit 12. The quartz oscillation circuit 4 outputs a 32,768-Hz signal on the basis of a vibration of the quartz oscillator. The time counting circuit 5 frequency-divides the 32,768-Hz signal output from the quartz oscillation circuit 4 and outputs a signal necessary for preparing a driving pulse or a signal at a one-second period which is a timing for rotating the stepping motor 13 to the pulse preparation circuit 107. The pulse preparation circuit 107 prepares driving pulses having various pulse widths as will be described later and outputs the driving pulses to the pulse selection circuit 108. The pulse selection circuit 108 selects only one appropriate driving pulse from the driving pulses having various pulse widths, which are prepared by the pulse preparation circuit 107, on the basis of a signal output from the rotation detection circuit 12 and outputs the driving pulse to the driver circuit 11. The driver circuit 11 drives the stepping motor 13 in accordance with the signal output from the pulse selection circuit 108. The rotation detection circuit 12 detects a rotation or non-rotation state of the stepping motor 13 and outputs the information to the pulse selection circuit 108. When the power generation voltage of the solar cell 1 exceeds 2.6 V, a discharge circuit (not shown) operates to prevent a voltage of 2.6 V or more, i.e., the breakdown from being applied to the electric double layer capacitor 2.
Driving pulses prepared by the pulse preparation circuit 107 will be described below.
FIG. 3 shows waveform charts of driving pulses prepared by the pulse preparation circuit 107. FIGS. 3a-3c show three driving pulses of the eight driving pulses, each having different pulse widths, which are prepared by the pulse preparation circuit 107. Each driving pulse is output at a timing of one second. FIG. 3d shows a compensation driving pulse which is also prepared by the pulse preparation circuit 107 and output when the timepiece load, i.e., the stepping motor 13 cannot be driven with the above driving pulses. The compensation driving pulse is a pulse having a width of 8 ms and output 30 ms after a normal driving pulse is output.
For the driving pulse shown in FIG. 3a of FIG. 3, a pulse width of 4 ms is divided into four equal portions 201a, 201b, 201c, and 201d. Each of the portions 201a, 201b, 201c, and 201d is further divided into 32 equal portions. The pulse is output during the first 28/32 period, and no pulse is output during the remaining 4/32 period (this is expressed as a "28/32 driving pulse"). Similarly, FIG. 3b shows a "26/32 driving pulse". In this prior art, 24/32, 22/32, 20/32, 18/32, 16/32, and 14/32 driving pulses are prepared additionally. That is, a total of eight driving pulses P.sub.4, P.sub.5, P.sub.6, P.sub.7, P.sub.8, P.sub.10, P.sub.12, and P.sub.14 are prepared, as shown in Table 1 below. As a matter of course, when the stepping motor 13 can be driven with these driving pulses, no compensation driving pulse is output.
TABLE 1 ______________________________________ Driving Pulses Minimum Driving Voltages ______________________________________ P.sub.4 28/32 1.24 V P.sub.5 26/32 1.33 V P.sub.6 24/32 1.45 V P.sub.7 22/32 1.56 V P.sub.8 20/32 1.74 V P.sub.10 18/32.sub. 1.92 V P.sub.12 16/32.sub. 2.14 V P.sub.14 14/32.sub. 2.43 V ______________________________________
Table 1 shows the eight driving pulses and their minimum driving voltages. For example, the minimum driving voltage of the "28/32 driving pulse" (driving pulse P.sub.4) is 1.24 V. This means that the driving pulse P.sub.4 can drive the stepping motor 13 at only a voltage of 1.24 V or more (of course, at or below 2.6 V, i.e., the breakdown voltage of the electric double layer capacitor) and cannot drive the stepping motor 13 at a voltage lower than 1.24 V. Similarly, the driving pulses P.sub.5, P.sub.6, P.sub.7, P.sub.8, P.sub.10, P.sub.12 and P.sub.14 have the minimum driving voltages as shown in Table 1, respectively. In FIG. 4, the minimum driving voltages for the driving pulses P.sub.4, P.sub.5, P.sub.6, P.sub.7, P.sub.8, P.sub.10, P.sub.12, and P.sub.14 are represented by small white dots. FIG. 4 shows a maximum charge voltage V.sub.MAX (actually 2.6 V) which is determined by the breakdown voltage of the electric double layer capacitor 2 constituting the power supply, and an operation limit voltage V.sub.L2 (actually 1.3 V) taking a calendar load and the like into consideration. The pulse widths of the driving pulses P.sub.4 to P.sub.14 are set to cover this voltage range.
As is apparent from FIG. 4, a driving pulse having a large pulse width has a low minimum driving voltage. To the contrary, a driving pulse having a small pulse width can drive the pulse motor at only a high voltage. In addition, the current consumption is minimized in driving at a voltage slightly higher (0.01 to 0.02 V) than the minimum driving voltage of each driving pulse. When the voltage becomes higher beyond that, the current consumption also increases.
When the motor is driven with a driving pulse having a certain pulse width, and the power supply voltage becomes higher than the minimum driving voltage of the next driving pulse having a pulse width larger than that of the present driving pulse by one level, the current consumption decreases in driving with the driving pulse having the larger pulse width. For example, the driving pulse P.sub.4 can drive the timepiece load at a voltage of 1.24 V or more. However, when the power supply voltage is 1.33 V or more, the current consumption decreases in driving with the driving pulse P.sub.5 having a pulse width smaller than that of the driving pulse P.sub.4 by one level. Therefore, within a power supply voltage range of 1.24 V to 1.33 V, the current consumption can be minimized in driving with the driving pulse P.sub.4. As for the remaining driving pulses as well, within a voltage range from the minimum driving voltage of each driving pulse to that of the driving pulse having a pulse width smaller by one level, the current consumption is minimized in driving with the driving pulse having the smaller pulse width.
An operation performed when the conventional electronic timepiece is driven using such a driving pulse will be described below.
Assume that the power supply voltage is 1.8 V. As is apparent from Table 1, when the power supply voltage is 1.8 V, the driving pulse P.sub.8 minimizes the current consumption. However, if the driving pulse P.sub.5 is output at this time, the current consumption excessively becomes large. Therefore, the driving pulse must be changed to the driving pulse P.sub.8 to decrease the current consumption. The method will be described below.
As described above, the driving pulse P.sub.5 is output, which has a sufficiently large driving force. Therefore, the stepping motor 13 is rotated, and the rotation detection circuit 12 detects the rotation state of the stepping motor 13 and outputs a rotation detection signal to the pulse selection circuit 108. Upon reception of this rotation detection signal, the pulse selection circuit 108 continuously outputs the driving pulse P.sub.5 as the next driving pulse. Similarly, the driving pulse P.sub.5 is continuously output during a predetermined period of time in this example, i.e., 200 seconds. After that, the driving pulse is finally switched to the "24/32 driving pulse" having a pulse width smaller than that of the driving pulse P.sub.5 by one level, i.e., the driving pulse P.sub.6. Subsequently, the same operation is repeated a number of times. After 200 seconds, the driving pulse is switched to the driving pulse P.sub.7 having a smaller pulse width. The "20/32 driving pulse" for allowing driving at a power supply voltage of 1.8 V and a minimum current consumption, i.e., the driving pulse P.sub.8 is finally set.
After driving with the driving pulse P.sub.8 for 200 seconds, the pulse selection circuit 108 switches to the "18/32 driving pulse" having a pulse width smaller by one more level, i.e., the driving pulse P.sub.10. As is apparent from FIG. 4, however, the driving pulse P.sub.10 has only a small driving force at a driving voltage of 1.8 V, so the stepping motor 13 cannot be driven and is set in a non-rotation state. The rotation detection circuit 12 detects the non-rotation state of the stepping motor 13 and outputs a non-rotation detection signal to the pulse selection circuit 108. As a result, the pulse selection circuit 108 immediately outputs the compensation driving pulse as shown in FIG. 3d, which has a sufficiently large driving force for driving the stepping motor 13, switches to the driving pulse P.sub.8 having a pulse width larger than that of the driving pulse P.sub.10 by one step, and outputs the driving pulse P.sub.8 as the next driving pulse. The driving pulse P.sub.8 is output for 200 seconds. During this time, the stepping motor 13 is continuously driven with the driving pulse P.sub.8, and this state is then maintained. Until this point of time, one driving pulse P.sub.10 and one compensation driving pulse are output. Although the current consumption of the compensation driving pulse is large, the compensation driving pulse is output every 200 seconds, so no problem of power consumption is posed. In this manner, a stable state is set by outputting a driving pulse suitable for the power supply voltage (1.8 V in the above example), and the current consumption can be kept small.
A case wherein the power supply voltage varies will be described below.
Assume that the power supply voltage increases to 2.2 V during driving with the driving pulse P.sub.8 at a power supply voltage of 1.8 V. As is apparent from Table 1, the driving pulse for minimizing the current consumption at a voltage of 2.2 V is the "16/32 driving pulse", i.e., the driving pulse P.sub.12. The pulse selection circuit 108 outputs the driving pulse P.sub.8 at a voltage of 2.2 V for 200 seconds, and thereafter, switches to the driving pulse P.sub.10 having a pulse width smaller by one level. After the driving pulse P.sub.10 is output for 200 seconds, the driving pulse is switched to the driving pulse P.sub.12 having a pulse width smaller by one level.
To the contrary, assume that the power supply voltage decreases to 1.6 V in driving with the driving pulse P.sub.8 at a power supply voltage of 1.8 V. In this case, driving with the driving pulse P.sub.8 so far is disabled. Therefore, a compensation driving pulse is temporarily output, and thereafter, the driving pulse is switched to the driving pulse P.sub.7 having a pulse width larger by one level.
In this manner, the driver circuit 11 can change the type of the output driving pulse to drive the load at the minimum current consumption even when the power supply voltage varies. The pulse preparation circuit 107 prepares the eight driving pulses capable of coping with the total voltage variation range of a predetermined power supply voltage. The above operation also copes with variations in timepiece load such as calendar updating.
For the electronic timepiece employing the above-described pulse width change driving technique, abnormal phenomena are known such that, when the voltage value for each driving pulse is excessively large, the stepping motor 13 causes a two-second skip or return by a reaction. A voltage which causes such an abnormal phenomenon will be referred to as an abnormality generation voltage. Abnormality generation voltages V.sub.04 (about 2.7 V) and V.sub.05 for the driving pulses P.sub.4 and P.sub.5 have values as shown in FIG. 4. However, as described above, in the electronic timepiece using a power supply formed of a combination of a solar cell and an electric double layer capacitor, the maximum charge voltage V.sub.MAX is designed not to exceed 2.6 V because of the breakdown voltage of the electric double layer capacitor. For this reason, the abnormality generation voltage V.sub.04 of about 2.7 V for the driving pulse P.sub.4 or the higher abnormality generation voltage V.sub.05 for the driving pulse P.sub.5 is not actually generated. Therefore, the abnormal phenomena of the stepping motor 13 do not occur.
However, as described above with reference to FIG. 1, the driving time of the timepiece, can be increased by adding the driving pulse P.sub.1 having a minimum driving voltage of 1.0 V, to the pulse group shown in FIG. 4. In this case, since an abnormality generation voltage V.sub.01 for this driving pulse P.sub.1 is 2.3 V which is lower than the maximum charge voltage V.sub.MAX, this voltage can be actually generated by the power supply. For this reason, if the power supply voltage becomes higher than this abnormality generation voltage V.sub.01 (2.3 V) due to some reason while the driving pulse P.sub.1 is selected, the stepping motor 13 cannot perform a normal rotation operation. This may cause an abnormal phenomenon such as a two-second skip or return by a reaction.
The present invention has been made in consideration of the above situation, and has as its object to provide an electronic timepiece such as a solar timepiece having a power supply constituted by a power generation element and a charge element such as an electric double layer capacitor, whose output voltage is not constant and varies within a certain voltage range to supply a power, wherein the electronic timepiece can cover a large width of the power supply voltage and increase the driving time.
The gist of the present invention to achieve the above object is characterized in that a plurality of driving pulse preparation means for continuously changing the pulse widths within the range of the varying power supply voltage are arranged, and the plurality of driving pulse preparation means are continuously operated.
The applicant has already proposed an electronic timepiece having a plurality of driving pulse preparation means in Japanese Unexamined Patent Publication No. 57-77984. However, unlike the electronic timepiece using a solar cell as a power supply, this technique is associated with an electronic timepiece capable of independently coping with a silver storage battery having a power supply voltage of 1.55 V and a lithium battery having a power supply voltage of 3 V. In this technique, as shown in FIG. 5, a driving pulse group A corresponding to the silver storage battery and a driving pulse group B corresponding to the lithium battery are independently prepared. The voltage level of the battery loaded in the timepiece is determined. In accordance with the determined level, either the driving pulse group A or the driving pulse group B is selected, and driving pulses having different pulse widths are output in accordance with variations in load.
On the other hand, according to present invention, an electronic timepiece using a power supply whose output voltage largely varies within a certain range, such as a power supply constituted by a combination of a solar cell and an electric double layer capacitor, copes with variations in voltage and variations in loads by switching the pulse width of the driving pulse. Therefore, the object is different from that of the technique disclosed in Japanese Unexamined Patent Publication No. 57-77984.